(1) Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) communication apparatus and an ATM cell forwarding control method and, more particularly, to an ATM communication apparatus having the function of avoiding occurrence of congestion in an ATM network and an ATM cell forwarding control method for avoiding occurrence of congestion.
(2) Description of the Related Art
As a personal computer is widely spreading and the use of the Internet is increasing in recent years, in an access network, bursty data traffic is increasing other than voice and image traffics. In consideration of future trends in access networks, as service classes aiming at avoiding a congestion state which can occur in an access network where traffic is heavy and at efficiently accommodating traffic, the ATM forum and ITU (International Telecommunication Union) propose ABR (Available Bit Rate) and GFR (Guaranteed Frame Rate).
In the ABR service, each of user terminals determines an ACR (Allowed Cell Rate) which can be used each time in accordance with the state of a network by setting a PCR (Peak Cell Rate) as the upper limit of a cell transmission rate and an MCR (Minimum Cell Rate) as the lower limit. A data source end system searches for an ACR of itself from RM cells inserted to a data cell train at predetermined intervals (or predetermined frequency according to a bandwidth used). In each of the RM cells, the value of the PCR of the source end system is set as an explicit cell rate (ER). Each of nodes on an ABR connection can revise the value in the ER field in a received RM cell downward to a cell rate which can be supported by the node. At a node where congestion occurs, by setting an explicit forward congestion indication (EFCI) bit of a passing data cell to “1”, the occurrence of the congestion can be notified to a destination end system.
The destination end system receives an RM cell and returns it to the source end system. In the case where the occurrence of the congestion in the network was notified by the explicit forward congestion indication (EFCI) bit in the data cell, the destination end system rewrites the congestion indication (CI) bit in the RM cell to be returned to the source end system to “1”. When there is no special change in the contents of the returned RM cell, the source end system increases the value of the ACR at a predetermined rate, thereby gradually bringing the transmission cell rate to the PCR. On the contrary, when the congestion indication (CI) bit in the received RM cell is “1”, the ACR is decreased at a predetermined rate (1/RDF: Rate Decrease Factor). If the value of the ER set by the source end system has been rewritten to a lower value, by decreasing the allowed cell rate (ACR) to the value designated by the ER, a break from the congestion state is attempted.
On the other hand, in order to efficiently accommodate the traffic, GFR guarantees the minimum cell rate for each of connections. When an unused bandwidth exists in a transmission line, GFR allows the unused bandwidth to be equally shared by a plurality of connections multiplexed on the transmission line. In the case where an ATM cell is discarded at the time of congestion, in the GFR service, it is proposed to discard the cells on a packet unit basis by expecting a data retransmitting function of an upper layer in communication between terminals, such as the TCP/IP (Transmission Control Protocol/Internet Protocol).